1. Field of the Invention
The present invention related generally to wireless communication systems and, more specifically, to enhancing the functionality of reference signals transmitted from a User Equipment (UE). The reference signals provide, in general, an estimate of a channel medium experienced by the UE at a given time.
2. Description of the Art
Several types of signals are supported for the proper functionality of a communication system. This includes data signals for conveying information content and control signals, which are transmitted from UEs to their respective serving Base Stations ((BSs) or Node Bs) in an UpLink (UL) of the communication system and from the serving Node Bs to the UEs in a DownLink (DL) of the communication system, for conveying information for processing the data signals. For example, control signals include positive or negative ACKnowledgement signals (ACK or NACK, respectively) that are transmitted in response to (correct or incorrect, respectively) data packet reception and are associated with a Hybrid Automatic Repeat reQuest (HARQ) process, i.e., HARQ-ACK and HARQ-NACK signals. Control signals also include Channel Quality Indication (CQI) signals that a UE sends to a Node B to provide information about DL channel conditions the UE experiences. Further, Reference Signals (RSs), also known as pilots, are typically transmitted to provide channel estimation and enable coherent demodulation for the transmitted data or control signals or, in the UL, to be used by the receiving Node B to measure the UL channel conditions that the UE experiences. The RS used for demodulation of data or control signals will be referred to as a DeModulation RS (DMRS), and the RS, which is typically wideband in nature, used for sounding the UL channel medium will be referred to as a Sounding RS (SRS).
A UE, e.g., a terminal or a mobile station, may be fixed or mobile and may be a wireless device, a cellular phone, a personal computer device, etc. A Node B is generally a fixed station, which may also be referred to as a Base Transceiver System (BTS), an Access Point (AP), or some other similar terminology.
UEs transmit signals conveying data or control information through a Physical Uplink Shared CHannel (PUSCH), and in the absence of PUSCH transmission, the UEs transmit control signals through a Physical Uplink Control CHannel (PUCCH). A UE receives signals conveying data information through a Physical Downlink Shared CHannel (PDSCH) and DL control signals are conveyed through a Physical Downlink Control CHannel (PDCCH).
A UE transmits data or control signals over a Transmission Time Interval (TTI), which may, for example, correspond to a sub-frame with a duration of 1 millisecond (msec).
FIG. 1 is a diagram illustrating a UL sub-frame structure for PUSCH transmission in a UL of a conventional communication system.
Referring to FIG. 1, a sub-frame 110 for PUSCH transmission includes two slots 120, each slot 120 including seven symbols. Each symbol 130 further includes a Cyclic Prefix (CP), which is used to mitigate interference due to channel propagation effects. Some symbols in each slot may be used for DMRS transmission or SRS transmission. For example, in FIG. 1, symbols 140 and 160 are used for DMRS transmission and symbol 150 is used for SRS transmission. Further, the second DMRS in the sub-frame, i.e., symbol 160, may or may not be transmitted with its negative value (scaled with “−1”), as will be described in more detail below.
The PUSCH transmission BandWidth (BW) includes frequency resource units, which will be referred to herein as Resource Blocks (RBs). In FIG. 1, each RB includes NSCRB=12 sub-carriers 170, also referred to as Resource Elements (REs). A UE may be allocated one or more consecutive RBs for PUSCH transmission and one RB for PUCCH transmission.
PUSCH transmission or PDSCH reception by a UE may be scheduled by a Node B dynamically through a respective Scheduling Assignment (SA) transmitted by the Node B using a Downlink Control Information (DCI) format in the PDCCH or through Semi-Persistent Scheduling (SPS). The DCI format informs a UE about a data packet transmission by the Node B in the PDSCH (i.e., a DL SA) or about a data packet transmission to the Node B (i.e., a UL SA) in the PUSCH. With SPS, a UE transmits or receives data packets at predetermined sub-frames.
FIG. 2 is a block diagram illustrating a conventional coding process of an SA at a Node B.
Referring to FIG. 2, a Medium Access Control (MAC) layer IDentity (ID) of the UE (or UE ID) masks a Cyclic Redundancy Check (CRC) of the SA information bits in order to enable the UE to identify that the SA is intended for it. The CRC computation 220 of the SA information bits 210 is performed and then the CRC is masked using the exclusive OR (XOR) operation 230 between CRC bits and UE ID bits 240, where XOR(0,0)=0, XOR(0,1)=1, XOR(1,0)=1, and XOR(1,1)=0. The masked CRC is appended 250 to the SA information bits, and channel coding (such as convolutional coding) 260 is performed. This is followed by rate matching 270 to the allocated PDCCH resources, and by interleaving and modulation 280. Finally, the SA is transmitted as a control signal 290. For ease of description, it is assumed that both the CRC and the UE ID have the same length, for example, 16 bits.
A UE receiver performs the reverse operations of the Node B transmitter to determine whether it has an SA assigned to it.
FIG. 3 is a block diagram illustrating a conventional decoding process of an SA at a UE.
Referring to FIG. 3, a received control signal 310 is demodulated and the resulting bits are de-interleaved 320. Rate matching 330, as applied at a Node B transmitter, is restored and followed by channel decoding 340. The SA bits 360 are then obtained after extracting the CRC bits 350, which are then de-masked by applying the XOR operation 370 with the UE ID 380. Finally, the UE performs a CRC test 390. If the CRC test passes, the UE concludes that the SA is valid and determines the parameters for signal reception (i.e., DL SA) or signal transmission (i.e., UL SA). If the CRC test does not pass, the UE disregards the received SA.
An example of a UL SA is provided in Table 1 below, in order to provide information about some of the Information Elements (IEs) typically included in a UL SA.
TABLE 1IEs of a UL SA DCI format for PUSCH TransmissionNumberInformation Elementof BitsCommentResource Allocation11Assignment of Consecutive RBsMCS5MCS LevelsNDI1New Data Indicator (synchronousHARQ)TPC2Power control commandsCyclic Shift3SDMA (maximum of 8 UEs)IndicatorHopping Flag1Frequency Hopping (Yes/No)CQI Request1Include CQI report (Yes/No)CRC (UE ID)16UE ID masked in the CRCTOTAL40
The first IE provides a Resource Allocation (RA) in terms of RBs. Single Carrier Frequency Division Multiple Access (SC-FDMA) is assumed where the signal transmission BW is contiguous. For an operating BW of NRBUL RBs, the number of possible contiguous RB allocations to a UE is 1+2+ . . . +NRBUL=NRBUL(NRBUL+1)/2 and can be signaled with ┌log2(NRBUL(NRBUL+1)/2)┐ bits, where ┌ ┐ denotes a ceiling operation that rounds a number to its next higher integer. For example, for NRBUL=50 RBs, the number of required RA IE bits is 11. In general, regardless of the transmission method, the UL SA is assumed to include an RA IE.
The second IE provides a Modulation and Coding Scheme (MCS). For example, the modulation may be Quadrature Phase Shift Keying (QPSK), Quadrature Amplitude Modulation (QAM)16, or QAM64 and the coding rate may take discrete values between 1/16 and 1.
The third IE is a New Data Indicator (NDI). The NDI is set to 1 when the UE should transmit a new Transport Block (TB) and is set to 0 when the UE should transmit the same TB as in a previous PUSCH transmission (synchronous UL HARQ is assumed).
The fourth IE provides a Transmit Power Control (TPC) command for PUSCH and SRS transmission power adjustments.
The fifth IE is a Cyclic Shift Indicator (CSI) indicating a Cyclic Shift (CS) for the transmission of a Constant Amplitude Zero Auto-Correlation (CAZAC) sequence used as a DMRS. As will be described below, using a different CS of a CAZAC sequence can provide orthogonal multiplexing of a respective RS.
The sixth IE, Hopping Flag, indicates whether frequency hopping applies to the PUSCH transmission.
The seventh IE, CQI Request, indicates whether the UE should include a DL CQI report in the PUSCH transmission.
In order for a Node B to properly determine RBs and MCS for PUSCH transmission from a UE, the Node B estimates a UL channel medium experienced by the UE (i.e., a UL CQI) over at least a part of the operating BW to obtain a respective Signal-to-Interference and Noise Ratio (SINR) estimate. This UL CQI is typically obtained by the Node B using an SRS transmitted by the UE.
FIG. 4 is a diagram illustrating a conventional SRS multiplexing method in a UL sub-frame. Specifically, FIG. 4 illustrates an SRS transmission occurring in a last sub-frame symbol of every 2 sub-frames 460, 465.
Referring to FIG. 4, UE1 and UE2 multiplex PUSCH transmissions in different BWs during a first sub-frame 401, UE2 and UE3 multiplex PUSCH transmissions in different BWs during a second sub-frame 402, and UE4 and UE5 multiplex PUSCH transmissions in different BWs during a third sub-frame 403. That is, UE 1 data 410 and UE 2 data 420 are transmitted in different BWs in the first sub-frame 401, UE 2 data 420 and UE 3 data 430 are transmitted in different BWs in the second sub-frame 402, and UE 4 data 440 and UE 5 data 455 are transmitted in different BWs in the third sub-frame 403. Accordingly, UE1, UE2, UE3, UE4, and UE5 respectively transmit DMRSs 415, 425, 435, 445, and 455. UEs with SRS transmission may or may not have PUSCH transmission in the same sub-frame and, if they co-exist in the same sub-frame, SRS and PUSCH transmissions may be located at different BWs.
It is assumed herein that the RS (DMRS or SRS) is constructed from CAZAC sequences. An example of such sequences is given by Equation (1).
                                              ⁢                                            c              k                        ⁡                          (              n              )                                =                      exp            ⁡                          [                                                                    j2π                    ⁢                                                                                  ⁢                    k                                    L                                ⁢                                  (                                      n                    +                                          n                      ⁢                                                                                          ⁢                                                        )                                            ]                                                          (        1        )            
In Equation (1), L is a length of a CAZAC sequence, n is an index of a sequence element, n={0, 1, 2, . . . , L−1}, and k is a sequence index. For CAZAC sequences of prime length L, the number of sequences is L−1. Therefore, an entire family of sequences is defined as k ranges in {1, 2, . . . , L−1}. However, the sequences for DMRS or SRS transmission are not only generated using Equation (1).
For example, as 1 RB is assumed to include NSCRB=12 REs, CAZAC-based sequences can be generated either by truncating a longer prime length (such as length 13) CAZAC sequence or by extending a shorter prime length (such as length 11) CAZAC sequence by repeating its first element(s) at the end (cyclic extension), although the resulting sequences do not strictly fulfill the definition of a CAZAC sequence.
Alternatively, CAZAC sequences can be generated through a computer search for sequences satisfying the CAZAC properties.
FIG. 5 is a block diagram illustrating a conventional RS transmission process. Specifically, FIG. 5 illustrates a DMRS or SRS transmission process at a UE, based on a CAZAC sequence.
The frequency domain version of a CAZAC sequence may be obtained by applying a Discrete Fourier Transform (DFT) to its time domain version. By choosing non-consecutive REs, a comb spectrum can be obtained for either the DMRS or for the SRS. The number of combs is referred to as the Repetition Factor (RPF). A comb spectrum is useful for orthogonally multiplexing (through frequency division) overlapping SRS transmissions with unequal BWs. Such SRS are constructed by CAZAC sequences of different lengths, which cannot be orthogonally multiplexed using different CS.
Referring to FIG. 5, a frequency domain CAZAC sequence 510 is generated, the REs in the assigned transmission BW 530 are selected by subcarrier mapping 520, the Inverse Fast Fourier Transform (IFFT) is performed 540, the CS 550 is applied, CP 560 and filtering 570 are applied, and the generated signal is transmitted 580. The UE also applies zero padding in REs where the DMRS or the SRS is not transmitted (not shown). For brevity, additional transmitter circuitry such as digital-to-analog converter, analog filters, amplifiers, and transmitter antennas, as they are known in the art, are not illustrated.
A Node B receiver performs reverse functions of the UE transmitter.
FIG. 6 is a block diagram illustrating a conventional RS reception process. Specifically, FIG. 6 illustrates reverse operations of those illustrated in FIG. 5.
Referring to FIG. 6, an antenna receives a Radio-Frequency (RF) analog signal and after passing processing units such as filters, amplifiers, frequency down-converters, and analog-to-digital converters (not shown) the resulting digital received signal 610 passes through a time windowing unit 620 and the CP is removed 630. Subsequently, the CS of the transmitted CAZAC-based sequence is restored 640, a Fast Fourier Transform (FFT) 650 is applied, the selection through controlling reception bandwidth 660 for the transmitted REs is performed by subcarrier mapping 665, and correlation by multiplying 670 with the CAZAC-based sequence replica 680 is applied. Finally, the output 690 is obtained, which can be passed to a channel estimation unit, such as a time-frequency interpolator (for a DMRS), or a UL CQI estimator (for an SRS).
Different CSs of a CAZAC sequence provide orthogonal sequences. Therefore, for a given CAZAC sequence, different CSs can be allocated to different UEs and achieve orthogonal multiplexing of the RS transmitted by these UEs in the same RBs. This principle is illustrated in FIG. 7.
FIG. 7 is a diagram illustrating conventional orthogonal RS multiplexing using different cyclic shifts of a CAZAC sequence.
Referring to FIG. 7, in order for multiple CAZAC sequences 710, 730, 750, and 770 generated correspondingly from multiple CSs 720, 740, 760, and 780 of a same CAZAC sequence to be orthogonal, the CS value Δ 790 should exceed the channel propagation delay spread D (including a time uncertainty error and filter spillover effects). If Ts is the duration of one symbol, the number of CSs is equal to └Ts/D┘ where └ ┘ denotes a “floor” operation, which rounds a number to its lower integer.
Multi-User Multiple-Input Multiple-Output (MU-MIMO) can substantially improve the spectral efficiency of a communication system. With MU-MIMO, PUSCH transmissions from multiple UEs share at least part of a BW. MU-MIMO is facilitated if a Node B can obtain interference-free estimates of a channel medium experienced by MU-MIMO UEs. This requires orthogonal reception for the respective DMRS. If the PUSCH transmissions from MU-MIMO UEs share exactly the same BW, orthogonal DMRS multiplexing can be obtained using different CS of the same CAZAC-based sequence. The CSI IE in a UL SA indicates the CS. However, if the PUSCH transmissions from MU-MIMO UEs do not share exactly the same BW, orthogonal DMRS multiplexing using different CS is not possible as the respective CAZAC sequences have different lengths. However, the application of Orthogonal Covering Codes (OCC) in a time domain to the DMRS transmission can also provide orthogonal DMRS multiplexing. For example, using the sub-frame structure illustrated in FIG. 1, which has 2 DMRS symbols, the OCCs can be {1, 1} and {1, −1}. As for the CS, the UL SA should indicate the OCC for the DMRS transmission in the PUSCH.
An SRS transmission BW may depend on a UL SINR experienced by the UE. For UEs with low UL SINR, a Node B may assign a small SRS transmission BW, in order to provide a relatively large ratio of transmitted SRS power per BW unit, thereby improving a quality of a UL CQI estimate obtained from the SRS. Conversely, for UEs with high UL SINR, the Node B may assign a large SRS transmission BW because good UL CQI estimation quality can be achieved from the SRS while obtaining this estimate over a large BW.
Several combinations for the SRS transmission BW may be supported, as shown in Table 2 below.
A Node B may signal a configuration c through a broadcast channel. For example, 3 bits can indicate one of the eight configurations. The Node B can then individually assign to each UE one of the possible SRS transmission BWs mSRS,bc (in RBs) by indicating the value of b for configuration c. Therefore, the Node B can multiplex SRS transmissions from UEs in the BWs mSRS,0c, mSRS,1c, mSRS,2c, and mSRS,3c (b=0, b=1, b=2, and b=3, respectively, in Table 2).
TABLE 2Example of mSRS, bc RBs values for UL BW ofNRBUL RBs with 80 < NRBUL ≦ 110.SRS BW configurationb = 0b = 1b = 2b = 3c = 09648244c = 19632164c = 28040204c = 37224124c = 46432164c = 56020Not Applicable4c = 64824124c = 74816 84
A variation in a maximum SRS BW is primarily intended to accommodate a varying PUCCH size. The PUCCH is assumed to be transmitted at the two edges of the operating BW and to not be interfered with by the SRS. Therefore, the larger the PUCCH size (in RBs), the smaller the maximum SRS transmission BW.
FIG. 8 is a diagram illustrating conventional multiplexing of SRS transmissions in various bandwidths. Specifically, FIG. 8 further illustrates the concept of multiple SRS transmission BWs for configuration c=3 from Table 2.
Referring to FIG. 8, the PUCCH is located at the two edges, 802 and 804, of the operating BW and a UE is configured SRS transmission BWs with either mSRS,03=72 RBs 812, or mSRS,13=24 RBs 814, or mSRS,23=12 RBs 816, or mSRS,33=4 RBs 818. A few RBs, 806 and 808, may not be sounded but this usually does not affect the Node B's ability to schedule PUSCH transmissions in those RBs, as the respective UL SINR may be interpolated from nearby RBs with SRS transmission. For SRS BWs other than the maximum one, the Node B also assigns the starting frequency position of the SRS transmission to a UE.
The SRS transmission parameters for each UE are assumed to be configured by the Node B through higher layer signaling, for example, through Radio Resource Control (RRC) signaling. These SRS transmission parameters may include the transmission BW, the comb (if the SRS has a comb spectrum), the CS, the starting BW position, the period (for example one SRS transmission every 5 sub-frames), the starting sub-frame (for example the first sub-frame in a set of 1000 sub-frames), and an indication of whether frequency hopping according to a predetermined pattern is enabled between successive SRS transmissions.
In order to satisfy a service quality that is largely independent of the UE location in a cell, Inter-Cell Interference Coordination (ICIC) based on soft frequency reuse for the allocation of RBs in adjacent cells can mitigate the inter-cell interference experienced by UEs located near the cell edge. The allocation of some RBs to each cell for exclusive use by cell-edge UEs can be through semi-static or dynamic network coordination, taking into account the distribution (location and/or transmit power requirements) and throughput requirements of cell-edge UEs.
FIG. 9 illustrates a conventional application of frequency-domain ICIC.
Referring to FIG. 9, a UL operating BW 910 is divided into 6 sets of RBs, with the first and fourth sets allocated to cell-edge UEs of cell 1 920, the second and fifth sets allocated to cell-edge UEs of cells 2, 4, and 6 930, and the third and sixth sets allocated to cell-edge UEs of cell 3, 5, and 7 940. The RB sets are not contiguous due to implementation reasons or to maximize frequency diversity. A Node B may use the RBs over the entire UL operating BW to schedule PUSCH from cell-interior UEs, but may only use the allocated sets of RBs to schedule PUSCH from cell-edge UEs.
FIG. 10 is a diagram illustrating a conventional heterogeneous network.
ICIC is beneficial in heterogeneous networks, as illustrated in FIG. 10, where a macro-cell served by a macro-Node B 1010 encompasses micro-cells served by respective micro-Node Bs 1020 and 1030. As the macro-Node B covers a larger area than a micro-Node B, a UE connected to the macro-Node B (macro-UE) may transmit its signals with substantially higher power than a UE connected to a micro-Node B (micro-UE). Macro-UEs can therefore cause significant interference to micro-UEs especially if they are both located near their cell edge.
With conventional SRS hopping methods, the SRS transmission hops over a maximum configured SRS BW (the SRS transmission with BW mSRS,b, b>0, hops over a BW defined by mSRS,0). This is clearly inefficient for ICIC as cell-interior UEs should transmit SRS over substantially the entire operating BW used for PUSCH transmissions and cell-edge UEs should transmit SRS only in a part of the operating BW. Even more importantly, for heterogeneous networks, allowing SRS transmission by macro-UEs near a micro-cell to hop over an entire operating BW can create significant interference to the SRS transmissions by micro-UEs. Therefore, it is beneficial to enable SRS hopping with non-maximum transmission BW only in parts of the maximum configured SRS transmission BW.
Frequency-domain scheduling can exploit frequency selectivity of a channel and PUSCH scheduling can be in parts of an operating BW where a respective SINR is optimized according to a scheduler metric (such as for example a proportional-fair metric). In order to enable PUSCH scheduling over non-contiguous parts of the operating BW, it is beneficial to enable simultaneous SRS transmissions over non-contiguous BWs. This does not impact the SRS multiplexing capacity and does not increase the SRS overhead assuming that the total BW of SRS transmission remains the same.
Therefore, a need exists for a method to enable SRS transmissions over non-contiguous BWs.
Another need exists for a method to enable hopping of SRS transmissions over a BW smaller than a maximum SRS transmission BW.
Additionally, a need exists for a method to enable a UL SA to indicate an OCC a UE should apply to a DMRS transmission in a PUSCH.